Determining the conversion efficiency of an exhaust gas catalytic converter

ABSTRACT

In order to determine the conversion efficiency of at least one component of a catalytic converter box comprising at least one SCR catalytic converter and an oxides of nitrogen storage catalytic converter for the aftertreatment of the exhaust gases of an internal combustion engine, wherein the catalytic converter box is associated at the downstream end with a NO x  sensor, at least for a limited period of time the internal combustion engine is operated such that a first of the two components does not affect the signal detected by means of the NO x  sensor and the conversion efficiency of the second component is determined depending on the signal of the NO x  sensor that is not affected by the first component.

BACKGROUND OF THE INVENTION

The invention concerns a method for determining the conversion efficiency of at least one component of a catalytic converter box comprising at least one SCR catalytic converter and an oxides of nitrogen storage catalytic converter for aftertreatment of the exhaust gases of an internal combustion engine, wherein the catalytic converter box is associated with a NO_(x) sensor at the downstream end.

The invention further concerns a computer program that can be stored in a control unit for controlling and/or regulating an internal combustion engine. The invention also concerns a control unit for controlling and/or regulating the operation of an internal combustion engine.

In order to reduce the NO_(x) emissions, exhaust gas aftertreatment components for converting NO_(x) into harmless emissions are used with internal combustion engines and especially with gasoline engines with lean burn modes and with diesel engines. The components that are currently mainly used are the oxides of nitrogen storage catalytic converter (NO_(x) Storage-Catalyst, NSC) and the SCR catalytic converter (Selective Catalytic Reduction).

The oxides of nitrogen storage catalytic converter stores the oxides of nitrogen (NO_(x)) present in the flow of exhaust gas. When the storage catalytic converter is sufficiently full, the stored oxides of nitrogen are converted into harmless molecules in a special operating mode of the internal combustion engine, which is referred to as the regeneration phase or as regeneration and in which the internal combustion engine is operated with a rich fuel mixture, and are removed from the storage catalytic converter. The NO_(x) storage of the storage catalytic converter depends on the current loading, the temperature and other parameters, for example the so-called space velocity. If the relative loading of the storage catalytic converter increases, then eventually the delivered NO_(x) material can no longer be stored to a sufficient extent. The part that can no longer be stored exits as so-called slip of the storage catalytic converter and is consequently emitted in the flow of exhaust gas.

The conversion efficiency of the oxides of nitrogen storage catalytic converter depends on the storage behavior. This is usually initially described by a model, which is then adjusted using measurements with a NO_(x) sensor for the storage catalytic converter concerned, which is referred to as adaptation. During the adaptation, a quality factor is calculated that describes the conversion efficiency of the storage catalytic converter. Using the quality factor, the amount of NO_(x) that can be stored and the time from which NO_(x) slip arises can be calculated. An accurate adaptation between the model and the measured sensor value is thus important in order to be able to use the storage catalytic converter optimally and to keep the oxides of nitrogen emissions low.

During the operation of the internal combustion engine, the storage catalytic converter must be diagnosed in order to recognize damage, for example owing to an excessive temperature.

An SCR catalytic converter also enables the NO_(x) present in the exhaust gas to be selectively reduced inter alia to nitrogen (N₂) in the presence of residual oxygen in the flow of exhaust gas by catalytic action, for example using ammonia that is fed in. The introduction of ammonia can be carried out actively by means of the targeted dispensing of a urea water solution into the flow of exhaust material or passively by means of an oxidation catalytic converter disposed upstream of the SCR catalytic converter, for example a three-way catalytic converter or an oxides of nitrogen storage catalytic converter, which produces NH₃ during an operating phase in which the internal combustion engine is operated with a rich fuel mixture.

Moreover, the SCR catalytic converter can store excess NH₃ in a zeolite to a limited extent. The absorption capacity of the catalytic converter depends here on the temperature of the catalytic converter. If the absorption capacity is exceeded, the excess NH₃ exits the catalytic converter as slip. The stored NH₃ in the SCR catalytic converter also enables the reduction of the NO_(x) at operating points at which no ammonia is introduced.

With a system of a combination of a SCR catalytic converter and an oxides of nitrogen storage catalytic converter, higher overall NO_(x) efficiencies are achieved. A combination of said catalytic converters is referred to below as a catalytic converter box, wherein the two components are not necessarily disposed in a common housing.

For monitoring the NO_(x) conversion rate or the conversion efficiency, NO_(x) sensors are usually used downstream of each individual component.

SUMMARY OF THE INVENTION

It is the object of the invention to determine the quality of the individual components of an overall system consisting of a SCR catalytic converter and an oxides of nitrogen storage catalytic converter, and especially the component of the NO_(x) conversion of the SCR catalytic converter, in order to be able to determine the ageing thereof. A possibility is also to be shown to enable the conversion efficiency to be determined with sufficient accuracy even if only one individual NO_(x) sensor is used.

The object is achieved by a method of the aforementioned type by operating the internal combustion engine at least for a limited period of time such that a first of the two components does not affect the signal detected by means of the NO_(x) sensor, and the conversion efficiency for the second component is determined depending on the signal of the NO_(x) sensor that is not affected by the first component.

According to the invention, consequently a state occurs in which at least one of the components, for example the SCR catalytic converter, is placed into a state in which it is not affected by the signal detected by the NO_(x) sensor. Here it should be noted that depending on the current operating mode of the internal combustion engine both NO_(x) (in a lean operating mode) and also NH₃ (in a rich operating mode) can be measured in the flow of exhaust gas by means of a NO_(x) sensor.

According to a first possible embodiment, a state is established in which the SCR catalytic converter does not affect the NO_(x) signal. This is achieved by a reduction of the NH₃ stored in the SCR catalytic converter. For this purpose, according to one version the internal combustion engine is operated at least for a limited period of time such that a reduction of the NH₃ stored in the SCR catalytic converter takes place. If no or almost no more NH₃ is being stored in the SCR catalytic converter, the conversion efficiency of the NO_(x) storage catalytic converter is determined depending on the signal of the NO_(x) sensor. In particular, the internal combustion engine can be operated such that the SCR catalytic converter reaches a temperature at which the stored NH₃ is released.

According to another possible embodiment, the internal combustion engine is operated such that a delivery of NH₃ to the SCR catalytic converter is at least substantially prevented and a reduction of the NO_(x) by means of the stored NH₃ takes place.

By means of the method described above, it is achieved that NH₃ is no longer stored the SCR catalytic converter, whereby the SCR no longer influences the NO_(x) conversion of the overall system. The signal of the NO_(x) sensor can thus now be used to determine the conversion efficiency of the oxides of nitrogen storage catalytic converter.

According to another aspect of the invention, the internal combustion engine is operated for at least a limited period of time such that saturation of the NO_(x) storage catalytic converter with NO_(x) occurs and, if no or almost no more NO_(x) can be stored, the conversion efficiency of the SCR catalytic converter is determined depending on the signal of the NO_(x) sensor. Here a state is consequently established in which the oxides of nitrogen storage catalytic converter is saturated and thus an input of NO_(x) passes through almost completely unconverted. In particular, if the oxides of nitrogen storage catalytic converter is installed upstream of the SCR catalytic converter, said state can also be established by selecting an engine operating mode in which NO_(x) raw emissions are produced, so that the storage catalytic converter is fully loaded. If the storage catalytic converter is installed in the flow of exhaust gas downstream of the SCR catalytic converter, it can become filled by operating the internal combustion engine in an operating mode in which the NO_(x) concentration in the flow of exhaust gas upstream of the catalytic converter box is so high that even for an optimally operating or a new SCR catalytic converter the NO_(x) contained in the flow of exhaust gas cannot be fully converted. The NO_(x) slip from the SCR catalytic converter is then stored in the oxides of nitrogen-storage catalytic converter until the same is full.

If the storage catalytic converter is fully loaded, an adaptation of the SCR catalytic converter can be carried out, because the SCR catalytic converter as the only component now influences the NO_(x) concentration in the flow of exhaust gas to the NO_(x) sensor. Thus a diagnosis of the efficiency of the SCR catalytic converter can now be carried out, for example by a comparison of the mass of NO_(x) introduced into the SCR in comparison with the mass of NO_(x) at the position of the NO_(x) sensor.

According to an advantageous development of the method according to the invention, the NO_(x) storage catalytic converter is loaded with NO_(x) at least until a NO_(x) slip is measured at the NO_(x) sensor. An operating mode is then selected in which a fully operational or at least sufficiently operational SCR catalytic converter would convert the entire NO_(x) concentration in the exhaust gas. The conversion efficiency of the SCR catalytic converter is then determined depending on the signal of the NO_(x) sensor.

Consequently, the storage catalytic converter is initially loaded here such that a slip occurs. In this case the process can be as previously described. Regeneration of the storage catalytic converter is usually activated in said load state, but is now suppressed or prevented. Instead, a state is established in the overall system in which a NO_(x) concentration upstream of the catalytic converter box is so high at its maximum that the NO_(x) would be completely converted with an optimal new SCR catalytic converter with sufficient stored NH₃. In the case of a SCR catalytic converter that is still converting well enough, no NO_(x) slip would now be measured at the NO_(x) sensor. If the SCR catalytic converter has aged however, and thus no longer has the property of converting the NO_(x) sufficiently well, then a NO_(x) slip is measured at the sensor downstream of the NSC. The ratio between the mass of NO_(x) flow upstream of the SCR catalytic converter and the mass of NO_(x) flow measured by means of the NO_(x) sensor downstream of the SCR catalytic converter can now be used for the calculation of the quality factor or the conversion efficiency of the SCR catalytic converter.

According to a further embodiment of the method, the NH₃ storage capacity of the SCR catalytic converter is concluded depending on the signal of the NO_(x) sensor. Said development uses the property of NO_(x) sensors of being able to measure NH₃ instead of NO_(x) for a rich fuel mixture. Further, with the NO_(x) sensor it can be determined whether a rich or weak fuel mixture is currently available. In order to be able to conclude the NH₃ storage capacity of the SCR catalytic converter for the case in which no NO_(x) sensor is present, the information about the NH₃ in the flow of exhaust gas upstream of the catalytic converter box could be calculated by suitable NO_(x) raw mass models and/or oxidation catalytic converter models. It should be noted here that when using a NSC as a NH₃ source upstream of the catalytic converter box, a high NH₃ peak arises through the low storage of the NSC, which may also have to be taken into account in the model being used.

Preferably moreover, NH₃ desorption owing to the NH₃ arising from the NO_(x) stored in the oxides of nitrogen storage catalytic converter is taken into account when determining the NH₃ storage capacity of the SCR catalytic converter. In particular, NH₃ desorption is to be seen a short time after the changeover from a lean mode into a mode with a rich fuel mixture.

Said desorption can for example be determined by integrating the NH₃ signal up to a defined time and adding to the NH₃ storage capacity, or by fitting the curve of the NH₃ signal in the region of the NH₃ desorption to the later profile. The NH₃ storage capacity of the SCR catalytic converter can thereby be determined sufficiently precisely from the difference of the NH₃ flowing into the SCR catalytic converter and the NH₃ flowing out from the NO_(x) storage catalytic converter.

Alternatively or additionally, it can be provided to inhibit NO_(x) desorption owing to NH₃ arising from the NO_(x) stored in the NO_(x) storage catalytic converter by operating the internal combustion engine before the detection of the signal of the NO_(x) sensor such that the stored NO_(x) in the NO_(x) storage catalytic converter is released. This can for example be achieved by a temperature increase before the measurement.

According to a preferred embodiment, the NO_(x) storage capacity of the oxides of nitrogen storage catalytic converter is determined with a preferably only slightly rich fuel mixture, in particular with a lambda value greater than 0.97, because the downstream oxides of nitrogen-catalytic converter can convert the rich components in the mixture (HC and CO) to harmless components (CO₂ and H₂O) by means of the present oxygen storage capacity and thereby only slight exhaust gas emissions arise. Furthermore, in said region of the composition of the mixture the NH₃ produced by the upstream oxidation catalytic converter is greatest.

If the oxides of nitrogen storage catalytic converter is disposed in the flow of exhaust gas upstream of the SCR catalytic converter, the NH₃ from the desorption of the oxides of nitrogen storage catalytic converter could pass into the SCR catalytic converter and cause errors in the results. This can be prevented by emptying the NH₃ store of the SCR catalytic converter and the NO_(x) store of the oxides of nitrogen storage catalytic converter, and then by operating the internal combustion engine with a rich fuel mixture NH₃ is produced in the flow of exhaust gas and the signal of the NO_(x) sensor is analyzed to determine the NH₃ storage capacity of the SCR catalytic converter. If discharging the catalytic converter is achieved by means of a temperature increase, the catalytic converter could thus be cooled to the desired temperature in connection with a homogenous phase, for example with a lambda value=1, without NH₃ and NO_(x) emissions and NH₃ could be produced by an operating phase with a rich fuel mixture, which would not be affected by the empty oxides of nitrogen storage catalytic converter.

The object of the invention is also achieved by a computer program that is stored in a control unit for controlling and/or regulating an internal combustion engine by conducting the method according to the invention if the computer program is executed in the control unit. The object is further achieved by a control unit for controlling and/or regulating the operation of an internal combustion engine by a computer program that is stored in the control unit and the method according to the invention is conducted if the computer program is executed in the control unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, possible applications and advantages of the invention are revealed by the following description of exemplary embodiments, which are described using the figures, wherein the features can be important to the invention both on their own and also in different combinations, without explicit reference again being made thereto. In the figures:

FIG. 1 shows a schematic representation of some components present in a motor vehicle that are configured to perform the method according to the invention;

FIG. 2 shows some process steps of a possible embodiment of the method according to the invention in a flow diagram;

FIG. 3 shows a further flow diagram with process steps of a possible embodiment of the invention;

FIG. 4 sows some process steps during the performance of the diagnosis of the SCR catalytic converter; and

FIG. 5 shows a flow diagram with some process steps that can be used for the diagnosis of the SCR catalytic converter by means of NH₃ information.

DETAILED DESCRIPTION

FIG. 1 shows schematically an internal combustion engine 1 with an induction system 2, a fuel feed 3 and an exhaust system 4. An oxidation catalytic converter 5 (for example a three-way catalytic converter) and a catalytic converter box 6 are disposed in the exhaust system 4. The catalytic converter box 6 comprises a SCR catalytic converter 7 and a NO_(x) storage catalytic converter 8. A NO_(x) sensor 9 that is connected to a control unit 11 by means of a data line 10 is disposed downstream of the catalytic converter box 6 in the flow of exhaust gas. The internal combustion engine 1 is also connected by means of data lines or a bus system 12 to the control unit 11 that controls or regulates the internal combustion engine 1 during the operation thereof.

A memory area 13 in which a computer program 14 is stored that is programmed to perform the method according to the invention is formed in the control unit 11.

Besides the arrangement shown in FIG. 1, according to the invention further embodiments are possible. For example, the arrangement of the SCR catalytic converter 7 and of the oxides of nitrogen-catalytic converter 8 is exchanged or an exhaust gas probe is also provided upstream of the catalytic converter box 6.

In FIG. 2 some process steps are shown that are implemented during the diagnosis of the oxides of nitrogen storage catalytic converter 7 according to one possible embodiment. The method starts with a step 20. In a step 21 the internal combustion engine 1 is operated such that the NH₃ stored in the SCR catalytic converter 7 is released, which can be achieved by raising the temperature of the catalytic converter. Alternatively, the NH₃ level can be passively lowered by a NO_(x) reduction with simultaneous inhibition of the NH₃ feed.

In a step 22 the release of the NH₃ is ended. In a step 23 the internal combustion engine is operated with a lean fuel mixture. In a step 24 the NO_(x) slip is determined. For this purpose, the NO_(x) component in the flow of exhaust gas upstream of the catalytic converter box 6 is determined and the NO_(x) concentration downstream of the catalytic converter box 6 is measured by means of the NO_(x) sensor 9. The difference of said values gives the NO_(x) slip.

In a step 25 the previously described adaptation is carried out by for example calculating a quality factor or the conversion efficiency of the oxides of nitrogen storage catalytic converter 8. Alternatively, the conversion efficiency can be used to diagnose the oxides of nitrogen storage catalytic converter 8.

FIG. 3 shows some process steps during the performance of the diagnosis of a SCR catalytic converter. The method starts in a step 30. In a step 31 the oxides of nitrogen storage catalytic converter 8 is loaded with NO_(x). For this purpose, a suitable operating mode of the internal combustion engine 1 is selected, in particular the internal combustion engine 1 is operated with a lean fuel mixture. In a step 32 a check is made as to whether the storage catalytic converter 8 is fully loaded. For this purpose, it is measured by means of the NO_(x) probe 9 whether NO_(x) is contained in the flow of exhaust gas downstream of the catalytic converter box 6. If this is the case, then in a step 33 the mass of NO_(x) in the flow of exhaust gas upstream of the catalytic converter box 6 is determined. This can be carried out by means of a suitable model. If there is a further NO_(x) sensor upstream of the catalytic converter box 6, then said value can be determined particularly accurately by means of said sensor.

In a step 34 the difference between the mass of NO_(x) upstream of the catalytic converter box 6 in the flow of exhaust gas and downstream of the catalytic converter box 6 is determined. In a step 35 the conversion efficiency of the SCR catalytic converter 7 is concluded from said difference.

FIG. 4 shows process steps of a further embodiment that is carried out during the diagnosis of the SCR catalytic converter 7. The method starts in a step 40. In a step 41 the oxides of nitrogen storage catalytic converter 8 is loaded with NO_(x). In a step 42 it is measured whether there is a NO_(x) slip, i.e. the storage catalytic converter 8 is loaded. This is achieved by analyzing the signal of the NO_(x) sensor 9.

In a step 43 the regeneration of the oxides of nitrogen storage catalytic converter 8, which would usually begin now, is suppressed because the oxides of nitrogen storage catalytic converter 8 is fully loaded.

In a step 44 a state is established in which a maximum NO_(x) concentration upstream of the catalytic converter box 6 is such that the NO_(x) would be completely converted in the case of an optimally working or a new SCR catalytic converter with sufficient stored NH₃. In the case of a passive SCR catalytic converter 7 that is still converting sufficiently well, no NO_(x) slip is now measured at the NO_(x) sensor in the step 45.

The quality is now determined in a step 46. If the SCR catalytic converter 7 has aged and consequently no longer has the property of converting NO_(x) sufficiently well, a slip measured in the step 45 is correspondingly assessed in the step 46 and the conversion efficiency is determined.

FIG. 5 shows some process steps during the diagnosis of the SCR catalytic converter 7, wherein NH₃ values are used that are detected by means of the NO_(x) sensor 9. The process starts in a step 50.

In a step 51 the amount of NO_(x) in the flow of exhaust gas upstream of the catalytic converter box 6 is determined. This can either be measured by means of a further NO_(x) sensor or calculated by means of a suitable NO_(x) untreated mass model and an oxidation catalytic converter model. In a step 52, the mass of NH₃ in the flow of exhaust gas downstream of the catalytic converter box 6 is determined by means of the NO_(x) sensor 9.

In a step 53, NH₃ desorption may be taken into account. This occurs as a result of NH₃ being able to arise from the NO_(x) stored in the oxides of nitrogen storage catalytic converter 8. The desorption can be determined by integrating the NH₃ signal downstream of the NO_(x) storage catalytic converter 8 until a defined time and adding the result to the NH₃ storage capacity.

Alternatively, a graph of the NH₃ signal can be calculated and fitted to the later profile in the region of the NH₃ desorption. However, it can also be provided to operate the internal combustion engine 1 in the step 53 such that the NH₃ desorption is suppressed. This is achieved by emptying the NO_(x) storage catalytic converter 8 or releasing the NO_(x) stored therein before the measurement of the NH₃ signal in the step 52, for which purpose for example, a temperature increase before the measurement is the aim.

In a step 54 the NH₃ slip is analyzed and the conversion efficiency of the SCR catalytic converter is determined depending on the result. 

1. A method for determining conversion efficiency of at least one component of a catalytic converter box (6) comprising first and second components, wherein the first component is at least one SCR catalytic converter (7) and the second component is an oxides of nitrogen storage catalytic converter (8), for aftertreatment of exhaust gases of an internal combustion engine (1), wherein a NO_(x) sensor (9) is disposed downstream of the catalytic converter box (6), characterized in that at least for a limited period of time the internal combustion engine (1) is operated such that one of the first and second components does not affect a signal detected by means of the NO_(x) sensor (9) and the conversion efficiency of an other of the first and second components is determined depending on a signal of the NO_(x) sensor (9) that is unaffected by the one of the first and second components.
 2. The method according to claim 1, characterized in that for a limited period of time the internal combustion engine (1) is operated such that a reduction of the NH₃ stored in the SCR catalytic converter (7) occurs and if no or almost no more NH₃ is stored, the conversion efficiency of the oxides of nitrogen storage catalytic converter (8) is determined depending on the signal of the NO_(x) sensor.
 3. The method according to claim 2, characterized in that the internal combustion engine (1) is operated for the limited period of time such that the SCR catalytic converter (7) reaches a temperature at which the stored NH₃ is released.
 4. The method according to claim 2, characterized in that the internal combustion engine (1) is operated such that a feed of NH₃ to the SCR catalytic converter (7) is at least substantially prevented and a reduction of NO_(x) is carried out using the stored NH₃.
 5. The method according to any claim 1, characterized in that the internal combustion engine (1) is operated at least for a limited period of time such that saturation of the oxides of nitrogen storage catalytic converter (8) with NO_(x) takes place, and if no or almost no more NO_(x) can be stored the conversion efficiency of the SCR catalytic converter (7) is determined depending on the signal of the NO_(x) sensor (9).
 6. The method according to claim 1, characterized in that the oxides of nitrogen storage catalytic converter (8) is loaded with NO_(x) at least until a NO_(x) slip is measured at the NO_(x) sensor, an operating mode is selected in which a fully operational or at least sufficiently operational SCR catalytic converter (7) would convert the entire NO_(x) concentration in the exhaust gas and the conversion efficiency of the SCR catalytic converter (7) is determined depending on the signal of the NO_(x) sensor (9).
 7. The method according to claim 1, characterized in that the NH₃ storage capacity of the SCR catalytic converter (7) is concluded depending on the signal of the NO_(x) sensor (9).
 8. The method according to claim 7, characterized in that NH₃ desorption owing to the NH₃ arising in the NO_(x) stored in the oxides of nitrogen storage catalytic converter (8) is taken into account when determining the NH₃ storage capacity of the SCR catalytic converter (7).
 9. The method according to claim 7, characterized in that NH₃ desorption owing to the NH₃ arising in the NO_(x) stored in the oxides of nitrogen storage catalytic converter (8) is suppressed by operating the internal combustion engine (1) before the detection of the signal of the NO_(x) sensor (9) such that the NO_(x) stored in the oxides of nitrogen storage catalytic converter (8) is released.
 10. The method according to claim 7, characterized in that the internal combustion engine (1) is operated with a rich fuel mixture, while the signal of the NO_(x) sensor (9) is analyzed for determining the NH₃ storage capacity of the SCR catalytic converter (7).
 11. The method according to claim 7, characterized in that the internal combustion engine (1) is operated with an only slightly rich fuel mixture, while the signal of the NO_(x) sensor (9) is analyzed for determining the NH₃ storage capacity of the SCR catalytic converter (7).
 12. The method according to claim 7, characterized in that the internal combustion engine (1) is operated with an only slightly rich fuel mixture, with a target lambda value greater than 0.97, while the signal of the NO_(x) sensor (9) is analyzed for determining the NH₃ storage capacity of the SCR catalytic converter (7).
 13. The method according to claim 7, wherein the oxides of nitrogen-storage catalytic converter (8) is disposed in the flow of exhaust gas upstream of the SCR catalytic converter (7), characterized in that the NH₃ store of the SCR catalytic converter (7) and the NO_(x) store of the oxides of nitrogen storage catalytic converter (8) are emptied and then by operating the internal combustion engine (1) with a rich fuel mixture, NH₃ is produced in the flow of exhaust gas and the signal of the NO_(x) sensor (9) is analyzed to determine the NH₃ storage capacity of the SCR catalytic converter (7).
 14. A non-transitory computer-readable medium storing a computer program (14) that when executed performs a set of functions for controlling an internal combustion engine (1), the set of functions comprising the steps of claim
 1. 15. A control unit (11) for controlling the operation of an internal combustion engine (1), the control unit (11) comprising: non-transitory computer-readable medium storing a computer program (14) that when executed in the control unit (11) controls an internal combustion engine (1) by performing the steps of claim
 1. 